This invention relates generally to gas treatment, and more specifically to purification of a gas stream such as a stream of hydrogen containing impurities.
A known (Pressure Swing Adsorption) gas purification system follows a cycle consisting of adsorption, stepwise depressurization, purging, and stepwise repressurization. In that system, hydrogen recovery is on the order of 75 to 85 percent, depending on the number of adsorbers, and hydrogen recovery is optimized by effectively utilizing the hydrogen stored in the adsorber vessel at the end of the cycle to repressurize other adsorber vessels and to provide hydrogen for purging. One vessel is on adsorption at any one time while the others are in various stages of regeneration. The cycles are staggered so that one adsorber at a time comes on the line when another completes its adsorption step. At the end of an adsorption cycle, the vessel is depressurized in a cocurrent direction in four steps--three separate pressure equalization steps with three different adsorbers and a purge step with a fourth. During this depressurization, only pure hydrogen is removed. Following the concurrent depressurization, further depressurization is carried out in a countercurrent direction, with the removal of some impurities. The adsorber is then purged at low pressure with pure hydrogen. The gases from the final depressurization step and purge step go to fuel.
More specifically, the P.S.A. system operates approximately as follows: A gas containing hydrogen 70%, carbon dioxide 20%, carbon monoxide 4%, methane 2% and the balance nitrogen is passed through the first absorbent vessels at a pressure of approximately 180 to 300 psig at 100.degree.-110.degree. F. Under these conditions, all of the components of the gas with the exception of hydrogen are strongly adsorbed by the solid granular material contained in the vessel. The pure hydrogen leaving the vessel passes through a valve system to the product line. After a short period of time the adsorbent becomes saturated with the various impurities and the incoming gas is switched to a second adsorbent field vessel where the process continues. In the meantime, the first vessel is connected through valve systems to other vessels in the train and the pressure reduced rapidly to permit expulsion of the adsorbed impurities. Up to ten vessels have been utilized in systems of this kind with the interconnecting valve train containing 20 to 40 elements.
It can be seen that the P.S.A. process is mechanically complex, and requires extensive valving and piping, which is unduly costly.